Metal halide high-pressure discharge lamps are used, among other applications, as light sources for theaters, and for film and television studios. The light should have a color temperature which is as close to daylight as possible. Additionally, the lamps should provide this light of essentially daylight color temperature throughout the lifetime of the lamp, and without degradation as the lamp is being used. This is important since frequently a plurality of lamps are used to illuminate a scene in a theater, film or television studio or the like; when one lamp burns out, replacing that one with a new one will change the mix of light, and hence the color rendition of the scene. If a continuous production is in progress of being recorded, changing one lightbulb may change the color rendition of the scene which is being recorded. This is annoying to the viewers.
The referenced Kul et al Pat. No. 3,654,506, the disclosure of which is hereby incorporated by reference, as well as British Patent 1,376,509, describe mercury vapor high-pressure discharge lamps which have halogen additives, particularly dysprosium and/or holmium and/or thulium halides. These lamps provide light with a spectral composition approximating daylight, and a light color of about 6000 K. These lamps work very well and have found substantial acceptance; like many lamps of this type which are on the market, however, they experience a drop in color temperature of 2 K. and more per operating hour. Thus, the lamps are suitable only over a comparatively short operating time with respect to the required color temperatures. After some time, the color temperature has dropped to such an extent that it can no longer be used to record scenes on film or for television.